As the load on the existing mobile radio systems is continuously increasing, it becomes more and more important to efficiently utilize the scarce frequency resources. A lot of effort is today put into planning the mobile radio cell structure, in order to optimize, given a certain amount of bandwidth, the number of connections with acceptable channel quality. How the available channels are allocated to existing and about-to-be-set-up radio connections is of uttermost importance for the overall performance of the system. Various methods for channel allocation are described in the publication “Channel assignment schemes for cellular mobile telecommunication systems: A comprehensive survey”, IEEE Personal Communications, June 1996, I. Katzela and M. Naghshineh.
An important group of channel allocation schemes are the schemes which base the channel allocation decision on measurements of the co-channel interference which would be experienced by a candidate channel if a connection were to be set up on the channel. However, decisions based upon such interference measurements only can lead to sub-optimal allocation. It is possible that allocation of a channel with low co-channel interference will result in a negative impact on already existing calls in the network, and hence in deterioration of the quality of such calls. Such deterioration will often lead to disturbing hand-overs of the deteriorated calls to other radio channels, which may in turn lead to further existing connections being adversely affected.
Attempts to address the problem of avoiding unwanted impact on already existing connections when allocating a new channel to a connection can be found in the literature. In WO97/32444, a method for allocating a frequency to a cell is described, where one criteria for selecting a certain frequency is that the frequency to be selected cannot cause third order intermodulation products when combined with any of the selected frequencies being used in that cell. However, the impact on existing radio connections of setting up a new radio connection extends far beyond the impact of third order intermodulation products on existing connections in the cell where the new connection is to be set up. A very important effect to consider is the impact of co-channel and adjacent channel interference, which is mainly to be taken in account of for channels used in cells other than the cell where the connection is to be set up. In U.S. Pat. No. 5,491,837 a method for frequency allocation is described where the frequency is selected which requires the lowest transmit power to obtain a certain carrier over interference (C/I) value. A set-up transmit power threshold is used in the method, which prevents users that would need a lot of power, and would therefore produce a high level of interference, from entering the system. This is a rather crude way of solving the problem, since no threshold value would be the optimal threshold value for all frequencies at all moments. Thus, the available channels cannot be utilized in an efficient way using this method.
It would therefore be very advantageous if a method of predicting the interference impact of the set up of a new connection on already existing connections could be found. An object of the present invention is to provide such a method.